1. Field of the Invention
The invention relates to electrical transformers, for example for use in the power distribution industry.
2. Description of the Prior Art
Electrical transformers are used in a variety of applications to step down (or step up) a voltage supply to a load. When large voltages are involved, problems can arise when a fault develops in the load resulting in a sudden demand for power. Typically, this is dealt with by including circuit breakers and the like but these are expensive and not always reliable.
In accordance with the present invention, an electrical transformer comprises a primary winding; first and second magnetic circuits magnetically coupled with the primary winding and through which magnetic flux is driven by the primary winding; first and second secondary windings each associated with a respective one of the magnetic circuits and electrically connected together in series opposition; and a short-circuited superconducting winding, hereinafter known as the xe2x80x9cfault current windingxe2x80x9d linked by the magnetic flux in the second magnetic circuit.
We have developed a transformer which incorporates automatic fault current limiting by making use of a closed or short circuited superconducting winding. In use, the secondary windings will be connected to a load and under normal conditions, when the load impedance is high enough not to draw excessive current, the superconducting fault current winding has current induced in it which opposes the magnetic flux linkage in the second magnetic circuit. The second secondary winding therefore has no electromotive force induced in it, and the output of the transformer is determined by the properties of the primary winding and the first secondary winding only.
In the event of a fault, such as a reduction in load impedance, or a short circuit in the output circuit, the current in the fault current winding will rise. When it exceeds the superconducting critical current, the shorted winding becomes resistive, and if the resistance is great enough, the induced electromotive force is not sufficient to drive a current through the fault current winding which opposes all the flux linkage. This allows an electromotive force to be induced in the second secondary winding, which opposes that of the first secondary winding. This effect therefore reduces the output voltage of the transformer so as to limit the current which can be drawn from it.
The use of a superconducting fault current winding to achieve fault-current limitation is desirable because the mechanism is fail-safe, very fast, has no moving parts and is self-sensing. Also, at larger ratings, this approach becomes much more commercially viable in comparison with conventional circuit breakers.
Although the transformer could be constructed from resistive windings, preferably the primary and secondary windings are superconductive.
In the case of transformers, there is a potential saving in eliminating the ohmic loss in the windings, provided that the refrigeration cost can be made small enough. Also, superconducting transformers could be more compact because of the higher current density in the windings and the elimination of coolant circulation and heat-exchange hardware.
By making all the windings superconducting, the transformer principle allows properties of the superconductor to be matched to the application so that the superconducting material can be used in a form which can be readily manufactured and is robust.
Most conveniently, the primary, secondary and fault current windings are housed in a common cryostat. This significantly reduces the refrigeration overhead compared with independent systems.
It is possible to utilize a single turn for the fault current winding although a coil having more than one turn could also be used.
Typically, the reluctances of the two magnetic circuits will be similar, so that the current in the fault current winding is proportional to the current in the primary winding, which is in turn determined by the load impedance.
The transformer can be conveniently designed with identical cross-sections and lengths of iron (or other magnetic material such as ferrite) in the two magnetic circuits. However, under normal conditions, the first (not coupled to the fault current winding) will carry more flux than the second, so that the iron will be operating over a different part of its B/H curve, so that the reluctance will be slightly different. So long as the iron is not saturated, this difference should not affect the operation of the device.